Growing at Glebelands - Lettuce

Name: Lettuce (Lactuca sativa)
Family (for rotation): Asteraceae
800-1200 sds/g. 1000 sds = 1g. 5-10g/100m²
Sowing depth 1.5cm, min sowing temp 5ºC & max sowing temp 25ºC – best between 15 and 18° C, seedling emergence 6-12 days
Seed life 3 yrs

Grown at Glebelands for headed crop and salad leaf mix.
Main problem is avoiding shortages and gluts – ie sowings tend to “catch up” in sunny conditions, and everyone in the UK has lettuces all at once, including home growers. Better price for early and late crops.

Varieties - lots

Some tried and tested varieties:

Kamalia (Tamar and Demeter) – red Batavia variety, good all year round, can be cut for heads or salad mix.
Red Sails (Demeter) – red loose leaf variety, but can also be cut as heads. Good in colder conditions, ie spring and winter, and ok in summer
Cocarde -red/green oakleaf – looks nice in mix, vigorous, good from spring onwards
Barba dei Frati (Seeds of Italy)– green oakleaf – have grown under cover in winter, good all year round
Lollo Rosso/ Green Lollo Rosso/Lollo Biondi/Green salad bowl/Red salad bowl (Tamar and Demeter) – loose leaf varieties, useful for salad mix. Bijou was best red loose leaf from Tamar, but sadly discontinued. Nb: Lollo Rosso types grow more slowly, but can be quite hardy

Nb Demeter seeds are produced Biodynamically, and sold in the UK by Stormy Hall

Sowing:

Successively sown, as lettuce has a short harvest interval (ie will bolt, especially in cold weather)
Can sow in Feb (depending on weather) to plant under cover in Mar for early salad leaves. Then sow mid-end Feb onwards to plant outside Apr onwards – fleece earliest lettuces.
Last sowings for outdoor planting usually end July, but if space under cover available, can make later sowings
Lettuce best grown fast, to avoid risk of disease

lettuce in module traysAt Glebelands, lettuce is sown in seed trays then pricked out into modules, then transplanted at around 4 weeks, at 3-5 leaf stage. Can be sown directly into modules, especially more expensive pelleted varieties. Lettuce is generally rarely direct sown, except intensively for salad mix.

It needs cool conditions to germinate, which can be a problem in summer – put trays in a cool, shady spot until germination

Planting and cultivation:

Lettuce is very pH sensitive & requires a pH of 6.5 to 6.8. The ideal site is fertile and moisture retentive – well finished compost is ideal, but too much nitrogen can lead to soft growth, prone to tip burn, rots, aphids and frost damage.

Best to plant in sheltered, sunny beds in spring, then can plant in shade for summer crops, then plant late summer crops in sun again.

Can use seaweed spray when growing & before planting out. Ventilate well & don’t leave wet overnight

Ideally plant into moist soil, and water well in, as lettuce – especially in modules - can wilt very quickly in warm dry conditions. Best to keep well watered until established, though don’t let soil surface stay wet for long periods in cool or damp conditions

Moist conditions can cause mildew, while fluctuations in water availability can cause tipburn or even bolting.

We plant through a planting board – a board the same width as the bed, with holes cut diagonally at the correct spacings (4 rows/bed at 1ft spacings) – it makes hoeing faster, as it can be carried out diagonally.

In warm moist weather close spacing can harbour slugs and lead to the spread of mildew – wider spacings allow leaves to dry & prevent fungal build up.

High density spacings rely on rapid growth to out compete weeds, while wider spacings allow for hoeing etc. Tighter spacing is used in slower growing period of winter. Equal spacing between plants and between rows produces more uniform heads

Best to hoe a few days after planting – depending on the weather - when plants small to avoid damaging large leaves & to avoid weed competition. Avoid getting soil into plants, and avoid pushing too much soil around base of plants as this can increase risks of rots and disease

Harvesting

Need to be cut when ready – may need to selectively cut. In hot conditions, may need to cut small and perhaps “double up” rather than risk plants bolting and becoming unsellable. It is also very important to cool immediately after cutting, especially in hot conditions – best to harvest early in the morning for this reason

Problems

Obviously, best to encourage healthy plant growth to minimise risk of pest and disease problems, and encourage the plant’s natural in-built general resistance

Downy mildew Bremia lactucae - yellow patches on upper leaf surface with greyish white marks on corresponding lower surface. Patches become brown rot. A fungus which thrives in moist, humid & cool conditions, but which can remain active on crops at any time of year. Maintain good hygiene, including removal of crop debris and keep plants generally healthy. Resistant varieties are also available.

Grey Mould Botrytis cinerea - a rot of the head or of the bottom leaves and stem – can occur in wet conditions, or on damaged or over-fertilised crops. Damage can occur at planting, or by tip-burn, frost or other pathogens. Avoid damaging plants when transplanting, maintain good crop growth, good drainage & hygiene

Bottom Rot Rhizoctonia solani caused when fungal mycelium in the soil come into contact with young basal leaves. Blackish lesions develop on undersides of leaves which become covered with a visible brown web of fungal strands. Avoid by rotation, hygiene and good ventilation

Bacterial rots can occur together with other rotting diseases – affected plants break off at ground levels, or leaves and stems can become discoloured and slimy. Avoid planting in wet conditions and overwatering

Sclerotinia rot sclerotinia sclerotiorum: affected plants collapse quickly and become covered with a fluffy white fungal growth in which black resting bodies (sclerotia) formed. Infected plants should be removed so sclerotia cannot return to the soil – debris from infected crops should not be incorporated into the soil. Ideally avoid growing lettuce on same site for min 5 yrs

Aphids
On leaves, can be avoided by keeping plants free of weeds which can harbour the aphids, turning in crop residues to prevent carry over of aphids and viruses, isolating new plantings from previous crops, using irrigation to remove aphids from foliage and prevent multiplication. Can use soft soap from 3 weeks after planting at weekly intervals before aphids can migrate to inner leaves, and encourage natural predators

Root aphids Pemphigus bursarius
Symptoms can be sudden wilting, particularly in dry conditions. The aphid overwinters in Lombardy and Black poplars and migrates to lettuce crops in June, so crops sown in June will escape infestation. Irrigation can help prevent damage, especially in dry years. Need good hygiene to prevent overwintering of colonies on lettuce debris, and can use resistant varieties

Cutworms
At Glebelands we had problems with cutworms in areas previously under grass. They are the caterpillar stage of some moths and may be active from mid-June onwards – grass and weeds can provide cover for the moths to lay their eggs on. Crops on light well drained soils are more prone to damage. Moths and young larvae die in wet soil so plants at risk need to be irrigated - extra cultivation prior to planting, and two very wet summers seem to have drastically reduced their numbers

Wireworms and Leatherjackets
These are the larvae of the click beetle and crane fly - common on fields just ploughed from grassland. Wireworms decline after several years while late planting can reduce leatherjacket damage as they are more active in spring. Cultivation can cause desiccation and also expose them to birds.

Slugs
High organic matter encourages slug survival but you can reduce food and shelter: don’t give them any cover by removing weeds, breaking down any large clods, not planting crops near long grass, hedgerows and banks can also help

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